This application relates generally to methods and systems for acquiring seismic data. More specifically, this application relates to methods and systems for acquiring seismic data without the need for wireline telemetry or radio-telemetry components or radio initiation.
Present-day land-based oil and gas drilling sites are selected from three-dimensional images produced through the use of reflection seismic data. The images are developed from data acquisition through active seismic tomography. Synthesized physical shock waves are applied to a survey site. These waves reflect off rock strata at variable velocities and return to the surface. Geophones at the surface measure and record the ground motion at the survey site. The seismic response from each receiver point (a geophone unit or the summed response of several geophone units) is collected centrally by a data collection center. The collected data are reduced through sophisticated computer analysis for producing three-dimensional maps of the geologic structure.
A typical seismic survey site can comprise an active receiver spread measuring 8 km2, with a plurality of receiver points located on a grid every 15–100 m. The seismic receivers are intended to respond to seismic events induced by human-generated explosives or mechanical sources. Accordingly, the receivers are typically configured to record data for time periods of about several seconds. In addition, the use of human-generated explosives limits the geographic distribution of the receivers since explosives often cannot be used within towns or cites, among other examples.
Examples of currently used modes for seismic recording include the following: (1) seismic data from each receiver channel are transmitted to a central collection unit via wires; (2) seismic data from each receiver are transmitted to the central collection unit via radio telemetry; and (3) data from each receiver channel are recorded in flash memory and downloaded later when each unit is connected to and processed by a mass storage device, such as a hard drive. Each of these modes has at least some disadvantages, a common one of which is the need for transmission of specific timing signals to the collection units to synchronize recording with the time of the human-generated seismic-vibration-inducing explosion. For example, while wire telemetry is reliable, quick, and allows examination of the collected data within seconds of recording, it requires the layout and maintenance of wires, which may frequently be disturbed, such as by animals or other sources of disturbance. Radio telemetry removes the need to maintain the wireline correction, but requires maintaining radio contact with all receiver units and the transmission of large amounts of data through shrinking commercial radio bands. Wireless telemetry is also slow and unreliable. The third mode removes some of the wireline connections, but still requires radio transmission of status and specific radio start-time synchronization information.
There is, accordingly, a general need in the art for improved methods and systems of acquiring seismic data.